Geology Reference
In-Depth Information
Figure 6.27. Distribution of bornhardts in respect of palaeosurfaces in Zimbabwe (Lister, 1976).
a geomorphic cycle which has attained an adv anced stage, or the time tak en to reduce a land
mass to baselevel. If, on the other hand, bornhardts are etch forms based in structural variations,
they can be of an y age; provided, of course, that the y can survive weathering and erosional
agencies.
Estimates of the duration of a c ycle vary widely. In par ticular, there is a major discrepanc y
between figures derived from fission track dating, and dates based in stratigraphy. Accepting,
pro
tempore
, stratigraphic dates, there is general agreement that even after due allowance is made for
isostatic recovery large areas of high land would be reduced to baselevel in periods of the order of
33-35Ma. Thus, no inselberg ought to be of greater age than this; in stratigraphic terms no inselberg
ought to predate the Oligocene, and most ought to be much younger.
